Heating, ventilation and air-conditioning (“HVAC”) systems are used in all types of commercial, industrial and residential facilities (hereinafter referred to as “buildings”). In general, the HVAC system is designed to maintain various predetermined set points, such as temperature. To that end, a system that generates hot air may be controlled on or off depending on the need for heat in a particular location. The supply of conditioned air (hot or cold) may further be controlled by the use of dampers within the air supply system. In larger buildings, the dampers may be actively controlled to regulate the supply of conditioned air.
HVAC systems thus include a wide assortment of components which interact with or influence other components. Accordingly, while various parameters on a particular device may be trending out of the normal operating range, the trend may be the result of a problem occurring in another component in the same or even a different system. Thus, while catastrophic failure of a component produces readily identifiable effects, a slowly developing problem is more difficult to identify.
Some efforts toward the early identification of potential problems include the monitoring of specific parameters. For example, much can be determined about a chiller's performance (and hence, required maintenance) by monitoring its water and refrigerant conditions and noting any deviation from design or established benchmark values. Basic chiller operating conditions include:                Condenser and evaporator pressure and corresponding temperature        Waterside temperature drop (ΔT)        a Waterside pressure drop (ΔP)        Heat exchanger approach temperature        Compressor discharge temperature        Purge unit run timeFIG. 1 illustrates design values, in I-P (SI) units, for an exemplary single stage R-123 centrifugal liquid chiller operating at full load conditions.        
The foregoing parameters can be used to identify many problems to which chillers are vulnerable. Many basic chiller problems result in a decreased heat transfer between the refrigerant circuit and the water circuit. In the condenser, or high side, this results in an elevated refrigerant pressure, while in the evaporator, or low side, the result is a lower refrigerant pressure. Increased condensing pressure and/or decreased evaporating pressure results in increased power consumption by the compressor motor and decreased system efficiency.
High head pressure (condenser pressure) is a standard safety cutout found on most chillers. Frequent chiller trips (i.e., when the chiller safety cutout is triggered, or “tripped”) can occur if the entering condenser water temperature habitually gets too high. Chiller trips due to hindered heat transfer within the condenser usually occur at less frequent intervals. An example of this is when the condenser water tubes gradually become fouled.
On the low side, if the compressor inlet guide vanes are inoperative or out of adjustment, the compressor may not match the evaporator load, causing elevated or lowered evaporator pressure and temperature.
The pressure in the evaporator or condenser, which in the exemplary single stage R-123 centrifugal liquid chiller are shell and tube heat exchangers, corresponds to a given temperature. At this temperature and pressure, the refrigerant, in a vapor/liquid state, is changing state as it releases heat to or absorbs heat from the circulating water. These temperature/pressure relationships are generally provided on the manufacturer's refrigerant chart for the particular refrigerant. The operating temperature/pressure may be useful in determining the health of the chiller.
While constant monitoring of a system is beneficial, particularly for rapidly escalating situations which have not yet pushed a parameter out of its normal range, merely monitoring the operating conditions do not provide the desired insight into more slowly developing problems. The identification of slowly developing problems is further complicated by the fact that the loading of equipment varies over time. For example, the temperature on a given day may be cool in the early hours of the day. Thus, systems used for cooling may not be operating at all. As the day progresses, however, the outside temperature may rise, driving temperatures in the building upwardly. In response, the cooling system initiates or works harder to maintain the water provided to the various terminals located throughout the building at a constant temperature.
Accordingly, the load on a cooling system may vary from zero to one hundred percent loading. As the load on the system changes, of course, the various operating parameters of the system will vary. This normal variance can mask developing problems.
In order to correct slowly developing problems before a component failure, it is common for many types of systems and/or equipment to be subjected to maintenance, calibrations and/or alignments at regular intervals. These preventative measures are very effective at early detection and subsequent avoidance of component or system failures. Of course, the procedures may be quite expensive. Moreover, depending upon the nature of the particular system, the capacity of the system may be curtailed or even eliminated during the foregoing activities. These considerations weigh toward a long period of time between the various preventative measures. As the time between the various activities increases, however, the chance of an undesired catastrophic failure also increases.
As a consequence, there is a need for apparatus and method that can reduce at least some of the drawbacks and costs identified above. For example, there is a need for a method and/or apparatus that reduces the costs associated with the determination of the health of a device. There is a further need for a method and/or apparatus that can be used to ascertain the health of a device or system that does not place undue constraints on the use of the equipment or system. There is yet a further need for a method and/or apparatus that provides insight into the health of the equipment or system which is not unduly affected by the normal variations in the operating parameters of the equipment or system.